for example any drive/partition with btrfs on it with heavily tweaked cfq would lead to errors and hang of the filesystem, similar behavior could be observed with ZFS

worst-example: external drive with ZFS on it - during backup the drive from time to time would suddenly (or even right from the beginning) timeout and/or shutdown (western digital red, I had similar experience with seagate barracude and ES.2 drives in the past)

so I started looking for new borders and arrived on deadline (or: went back to the roots):